A direct drive wind turbine generator is a wind turbine generator in which a main shaft transmitting rotation of the wind turbine rotor directly rotates the generator rotor within the generator. In an ordinary wind turbine generator, the rotation frequency of the wind turbine rotor is lower than the frequency of the utility grid, and therefore a gear box is used.
In a direct drive wind turbine generator of the system, on the other hand, the main shaft joined to the wind turbine rotor is directly connected to the generator without using a speed-up gear, and the size of the generator tends to be increased in the wind turbine generator of this type. This is because the rotation frequency of the wind turbine rotor does not match with the frequency of the utility grid and therefore the number of poles of field magnets within the generator must be increased. When the number of poles of the field magnets is increased, the diameter of the generator rotor is also increased, and this results in an increase in the size of the generator. Further, the size of the generator tends to be further increased due to the recent advance of the capacity.
It is one of important matters in designing a direct drive wind turbine generator to optimally design a mechanism for supporting a generator, which tends to grow in size. In the direct drive wind turbine generator, two mechanisms are generally used as the generator supporting mechanism.
First, a bearing is provided between the main shaft and the stator casing, and the stator casing is supported by the bearing. The reason why such a mechanism is used is to maintain the size of the gap between the stator and rotor in the generator. In a direct drive wind turbine generator, a deflection occurs in the main shaft due to the gravity acting on the wind turbine rotor provided at the distal end thereof and the gravity acting on itself. Here, the generator rotor coupled to the main shaft is also displaced according to deflection of the main shaft. In order to maintain the size of the gap between the stator and the rotor even in a case that a displacement of the generator rotor occurs, the stator casing is only required to be supported also by the main shaft.
On the other hand, the approach in which the stator casing is supported by a bearing provided on the main shaft cannot support torque acting on the main shaft in the circumferential direction. Especially, when the generator rotor rotates, torque forcing the stator casing to rotate about the main shaft is applied to the stator casing by the attraction force acting between the generator rotor and the stator. Such torque cannot be supported by the bearing provided on the main shaft.
To address this, a mechanism for coupling a static member (typically, a base on which bearings supporting the main shaft are mounted) and the stator casing is provided to support the torque forcing the stator casing to rotate about the main shaft. Such a mechanism is called “torque support” in this description. The torque support is a mechanism which supports torque acting on the stator casing in the circumferential direction of the main shaft to prevent the rotation of the stator casing.
A structure for supporting a generator by the bearings provided on the main shaft and the torque support is disclosed, for example, in Patent Literature 1 (EP1327073B1) and Patent Literature 2 (EP2014917A1). FIG. 10 is a perspective view showing the structure of a wind turbine generator disclosed in the patent literature 1. In the structure shown in FIG. 10, an arm 226, a beam 227, and a damping element 228 are provided on the face of a stator 219, and the beam 227 is attached to a base 204. Such a mechanism functions as a torque support which supports torque forcing the stator 219 to rotate about the main shaft. In addition, bearings 216 and 217 are provided on the main shaft 218, and the stator 219 is also supported by these bearings.
On the other hand, in a wind turbine generator disclosed in the Patent literature 2, the main shaft is supported by first and second bearings provided on a base plate. A stator casing includes a first end plate and a second end plate provided around a rotor, and a casing element. Further, the first end plate is supported by a third bearing provided on the main shaft. The second end plate is attached to a bearing housing by deformable non-rotatable coupling. The non-rotatable coupling functions as a torque support which supports torque forcing the stator casing to rotate about the main shaft.
The inventor is advancing investigations about the generator supporting mechanism, and according to findings obtained by the inventor, one of important matters in designing the torque support is to reduce a bending moment acting on the stator casing (namely, force deflecting the stator casing out of plane). When the stator casing is deformed due to the bending moment acting on the stator casing, the gap between the stator and the rotor cannot be kept constant. This undesirably causes an increase in vibrations of the generator and a degradation of the generator performance. Especially, when the structure of the torque support is improper, a large bending moment undesirably acts on the stator casing. In the structure shown in FIG. 10, for example, the bending moment undesirably acts on the stator casing, since the arm 226 is joined onto the surface of the stator 219. Especially when the output power of the generator is increased, the torque acting on the stator casing in the circumferential direction is also increased and the bending moment is also increased, and this results in that the problem related to the bending moment becomes severe.
One measure to address the problem of the bending moment may be increasing the rigidity of the stator casing to resist against the increase of the bending moment. Such measure, however, undesirably increases the weight of the stator casing increases. Another measure to address the problem of bending moment may be supporting the stator casing as outside as possible. In the structure shown in FIG. 10, for example, the arm 226 and the beam 227 are provided so as to extend to the vicinity of the outer periphery of the stator 219 to thereby support the stator 219. However, the extension of the arm 226 and the beam 227 to the vicinity of the outer periphery undesirably causes size enlargement and weight increase of the arm 226 and the beam 227.